Firstly, Scots is not a form of English, its actually just as closely related to Scandianvian languages and Dutch, but has been under English influence for so long that you really have to look deep to see it. There's a really interesting article here: http://www.omniglot.com/writing/scots.htm

From my experience of being a foreigner in Scotland, I can only agree with Skauwt. My impression is that all over the lowlands, and on the islands, those not speaking Gaelic (which are only a few) speak scots most of the time. Apart from in the larger cities, that is. I have no difficulty believing that a third of the population speak Scots.

I think the situation is somewhat comparable to Norway before WW2 - everyone spoke Norwegian, apart from the aristocracy and the church officials, but everyone wrote Danish. Eventually the written language seeps over into the spoken, and you "water out" the spoken language.

OT:
Norway now has two written languages, one based on Danish and one based on spoken Norwegian. Only problem being that with over 200 quite distinct dialects, you get to the point where you have to choose which dialects not to include in the written. For instance, the simple word "I" can be very different from one area to the other (jeg, eg, ek, i, e, æh, je), and two have been chosen as written forms. Some linguists have argued that the differences between western Norwegian and northern Norwegian (dialects of same language) are greater than those between Bosnian and Croatian (separate languages).

BACK ON TRACK:
This is what happens to Scots now, hopefully they'll start teaching Scots in schools so that a proud people can reclaim their beautiful language.